If you're a good coder you could do something like this.

Setup a website that gives away free ringtones. The user will enter their number to receive their free ringtone. Your site/script takes their number over to Craigslist and creates a new account, cracks the CAPTCHA and enters the users phone number. On your ringtones site you will ask the person to enter the code they received from the automated call to get their free ringtone. They enter the phone number and your script enters it over on Craigslist.

Then send the user to your ringtone affiliate link